Which choice most logically completes the text?

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Microbial fuel cells (MFCs) capitalize on the ability of some species of bacteria to metabolize metal, liberating electrons. The bacteria form a dense biofilm on the surface of an electron-collecting anode, but moving the electrons from the bacterial cytoplasm to an external electrode requires that the electrons pass through a series of inefficient oxidation-reduction (redox) reactions. Accordingly, MFC power output rarely exceeds a density of 0.30 milliwatts per square centimeter (mW/cm²). In an experiment, researchers added silver nanoparticles to carbon paper covering the anode in an MFC. The resulting power density was 0.66 mW/cm². Since metals such as silver exhibit high electrical conductivity, the researchers hypothesized that ________

silver nanoparticles may allow electrons to bypass the series of redox reactions and transfer directly to the electrode.

as the density of the biofilm increases, the series of redox reactions may accelerate independent of the presence of the silver nanoparticles.

electrons may be conducted directly to the electrode before the silver nanoparticles catalyze the redox reactions.

silver nanoparticles may increase the metabolic processes of the bacteria, thereby increasing the number of free electrons available to transfer to the electrode.